The present invention relates to a method and apparatus that are used to regulate by way of a regulating device the hydraulic fluid pressure that is applied through a control valve to one or more piston/cylinder units of a continuously variable cone-pulley transmission in order to set or change the transmission ratio.
A continuously variable cone-pulley transmission of the kind that the invention relates to has two adjustable pulleys, i.e., a first pulley mounted on a first shaft (input shaft) and a second pulley mounted on a second shaft (output shaft), and an endless flexible element such as a chain or belt (subsequently referred to as a chain-belt) to transmit torque between the pulleys. Each pulley is essentially a pair of conically tapered discs. In each pulley, one disc is rigidly connected to the respective shaft, while the other disc is constrained to the shaft only in the rotational sense but movable in the axial sense of the shaft.
The gripping pressure to hold the chain-belt between the discs is applied through actuating members that are arranged at the movable discs. The actuating members are pressurized by a torque sensor that is responsive to the magnitude of the torque to be transmitted. At least one of the disc pairs has a second actuating member configured as a ratio-controlling piston/cylinder unit that can be hydraulically pressurized to a pressure level (subsequently called the ratio-controlling pressure) that varies depending on the rpm-ratio (subsequently called the transmission ratio) that is to be set between the input shaft and the output shaft of the continuously variable transmission. The pressurized working fluid is delivered to the ratio-controlling piston/cylinder unit by way of a transmission-ratio valve.
The ratio-controlling pressure can be influenced by a pressure valve that supplies the transmission-ratio valve with an adequate amount of pressure for a rapid shifting of the axial positions of the movable discs and thus a rapid change of the D transmission ratio. For example, if the driver of a vehicle equipped with this type of transmission depresses the gas pedal to increase the speed of the vehicle, the immediate consequence is an increase in the amount of power that the engine delivers to the cone-pulley transmission. To translate the power increase into an acceleration of the vehicle requires an appropriate change of the transmission ratio, which requires a change in the ratio-controlling pressure that is delivered to the ratio-controlling piston/cylinder unit.
The transmission-ratio valve can be a pilot-controlled pressure-reducing valve whose pilot pressure is variable under the control of a proportional valve. The proportional valve, by generating appropriate changes of the pilot pressure, causes the transmission-ratio valve to open or to further increase the opening of a hydraulic passage to the piston/cylinder unit. The change of the pilot pressure delivered to the transmission-ratio valve occurs as a result of a change in the controlling electric current of the proportional valve. The controlling current of the proportional valve therefore has to be tied into a servo loop in which the actual current value is compared to a given target value, so that a deviation of the actual value from the target value can be corrected by the regulating servo. The same applies also in case where the transmission-ratio valve is controlled directly by an electric current to effect variations in the ratio-controlling pressure delivered to the piston/cylinder unit.
The vehicle power plant consisting of the continuously variable transmission and the combustion engine is exposed to significant variations of its operating conditions. For example, there may be large variations in the fluid temperature of the hydraulic circuit. As another example, a change of the transmission ratio will also cause changes in the behavior of the piston/cylinder unit and in the regulating behavior of the transmission-ratio valve. These are only two examples of factors that have an influence on the behavior of the hydraulic control of the transmission ratio.
It is therefore the object of the present invention to provide a method and apparatus for controlling the ratio-controlling pressure of a working fluid by means of a regulating device, wherein variations in the regulating behavior of the transmission-ratio valve under different operating conditions are taken into account.
The invention meets the foregoing objective by introducing a method of regulating by way of a regulating device the ratio-controlling pressure that is applied through a transmission-ratio valve to one or more piston/cylinder units of a continuously variable cone-pulley transmission in order to set or change the transmission ratio. According to the invention, the ratio-controlling pressure is regulated in such a way that certain operating parameters of the regulating device (that will be referred to as control parameters) are adjusted depending on the amounts of ratio-controlling pressure to be applied and that the regulation is performed by using the adjusted control parameters.
Thus, the inventive method takes into account that with changing operating conditions, the transmission-ratio valve can in some cases have significantly different response times when there is a change in a control variable or an abrupt change in an extraneous interference quantity. In other words, the transfer function of the transmission-ratio valve is subject to change under different operating conditions. When the amount of current is changed in the proportional valve that generates the pilot pressure for the transmission-ratio valve, or also when the amount of current is changed in a current-controlled transmission-ratio valve in order to change the position of the valve piston in the piston bore, the reaction times for moving the piston to a different position will vary, i.e., the settling times for transient oscillations will be of different length. If the regulation is performed, e.g., by means of a regulating device configured as a P/I servo with proportional and integrating components and constant control parameters, it is possible that certain deviations or excursions of a quantity to be regulated can no longer be brought under control because the stability limit of the servo circuit has been reached and the aforementioned oscillations would no longer settle down.
In a continuously variable cone-pulley transmission of a kind where both of the disc pairs on the input shaft and the output shaft, respectively, are equipped with a piston/cylinder unit for setting or changing the transmission ratio, there is a change-over in the pressure between the piston/cylinder units and in the transmission-ratio valve, so that the characteristic curve of the transmission-ratio valve (pressure vs. current) has an inversion point or area (zero-pressure range) where the transmission-ratio valve is pressure-free. Therefore, if the regulation were performed with constant control parameters throughout the entire range of the characteristic curve of the transmission-ratio valve, the regulating device would reach its stability limit when operating in the zero-pressure range, and the response would no longer be a transient oscillation settling in at the new target pressure. If the control parameters were adapted only according to a limited range in the vicinity of the inversion point, then the regulating device would have a markedly sluggish reaction when operating outside of the zero-pressure range. On the other hand, if the control parameters were dimensioned to work for a regulation outside of the zero-pressure range, the regulation would be unstable when operating inside the zero-pressure range, i.e., the regulating servo circuit would suffer from instabilities.
The invention solves the foregoing problem by introducing the concept of adapting the control parameters in the regulating device to the targeted ratio-controlling pressure and performing the regulation process by using the adapted control parameter values. Values for the control parameters are therefore determined for different operating points within the range of operating conditions of the transmission-ratio valve, to allow the regulating device to work optimally within its entire operating range by adapting the control parameters to the different operating points.
According to the invention, the control parameters are kept in the form of characteristic curves or as a field of characteristic curves, or also as equations to compute the control parameter values based on different input parameters that are relevant for the ratio-controlling pressure, such as the engine-rpm rate, transmission ratio, engine torque, and output torque of the continuously variable cone-pulley transmission. Now, when the transmission is in operation and as the regulating device enters one of the regulating ranges at the different operating points, the regulating device will regulate the ratio-controlling pressure by using the control parameter values that are applicable to that range based on the characteristic curves or the characteristic curve field or the aforementioned equations, which provide the control parameter values on the basis of measured or calculated values of relevant input parameters. It is also possible to use standard parameter values and adjust them by a certain correction factor depending on which part of the range of the transmission-ratio valve is being used, and to perform the regulation based on the corrected control parameter values.
In a further developed version of the inventive method, the control parameters are determined depending on the amplification or gain factor of the transmission-ratio valve and the regulation is performed with parameter values that are corrected in the inverse sense of the gain of the transmission-ratio valve. This is advantageous in a case where a change in the transfer function of the regulating valve can be described in terms of a change in the amplification of the valve, e.g., if the valve has a markedly lower amplification in the zero-pressure range than in the two ranges adjoining the zero-pressure range. Advantageously, the amplification is expressed in the form of an amplification curve and the correction that is applied to the control parameters is a multiplication by the inverse value of the amplification, so that the non-linear behavior of the ratio-controlling member can thereby be linearized.
The working fluid in the hydraulic circuit of the cone-pulley transmission can be an hydraulic oil of a temperature-dependent viscosity. The invention therefore includes the feature of determining the control parameters depending on the temperature of the working fluid.
The latter concept is advantageous in a case where the transfer function of the transmission-ratio valve changes as a function of temperature. For example, if the dynamic behavior becomes weaker with increasing temperature, it is advantageous to determine the control parameters dependent on the temperature or to apply a temperature-dependent correction to the control parameters, so that the control parameter values are lowered when there is a weakening of the dynamic response, whereby the regulating servo circuit can be prevented from becoming unstable as would be the case with control parameters of a fixed, constant value. Further under this concept, it is of advantage if the control parameters are expressed as a temperature-dependent curve. The parameter values used for the regulation could be standard values that are corrected by a certain percentage in accordance with the temperature-dependent curve so as to avoid the unfavorable regulating conditions that occur with fixed control parameters. The piston/cylinder unit at one disc set or the units at both disc sets are supplied by way of the transmission-ratio valve with a ratio-controlling pressure to set or change the transmission ratio. The transmission-ratio valve is part of a hydraulic circuit that is pressurized by a pump pressure or system pressure. The transmission-ratio valve receives this system pressure as input pressure and delivers a ratio-controlling pressure to the piston/cylinder unit. A further developed embodiment of the invention provides therefore that the control parameters be determined as a function of the ratio between the input pressure and the ratio-controlling pressure. If the regulating behavior of the transmission-ratio valve is changing as a result of a pressure differential between the system pressure and the ratio-controlling pressure, it is advantageous if this is taken into account in the regulation process and the control parameters used for the regulation are adjusted accordingly. The dynamic behavior of the transmission-ratio valve can become stronger with an increase in the pressure differential, and thus it will be advantageous to apply a corresponding upward correction to the control parameter values in order to shorten the settling time at the new target pressure.
As has been mentioned above, it is advantageous if the control parameters are expressed in the form of characteristic curves and/or characteristic curve fields and/or equations for the control parameters. According to an embodiment of the invention, a finite number of control parameter values are stored as table values which are used to interpolate intermediate values. This has the advantage that changes of the control parameters occur as continuous variations rather than in steps, so that there are no discontinuous changes that could adversely affect the comfort level of a vehicle equipped with a cone-pulley transmission that operates according to the present invention.
According to a further developed version of the inventive method, the changes applied to the control parameters between two regulating cycles, i.e., the change that the regulating device calculates in an individual step for the regulating process, is limited to a preset amount. This limits the rate of change between two regulation cycles, so that no abrupt changes will adversely affect the comfort of the occupants of the vehicle.
In accordance with the invention, the regulating device is a P/I servo (i.e., a feedback controller with a proportional and an integrating component) whose control parameters can be adjusted individually or together. If the transmission-ratio valve shows a delayed response in a regulation cycle because the valve piston has to overcome a static friction force in the bore hole, or if the valve has a markedly weaker response at the transition from static friction to sliding friction, the situation can be corrected according to the invention by adjusting only one of the control parameters, e.g., the I-component while leaving the P-component unchanged or, alternatively, by changing the P-component and leaving the I-component unchanged.
The invention further includes a method of regulating the transmission ratio of a cone-pulley transmission so that the transmission-ratio stays essentially constant.
For example, if a vehicle with a cone-pulley transmission is to maintain its speed at a level that can be given, e.g., by the driver, the transmission ratio will have to be kept at a constant level, if the rpm-rate of the engine is to stay unchanged. This is particularly important in the slow range or underdrive range of the transmission, because a change in the transmission ratio would lead to a higher vehicle speed. It is of special importance when a vehicle with a cone-pulley transmission moves backwards, that the transmission ratio stays unchanged when starting up and when traveling in reverse gear.
The transmission referred to in the preceding paragraph is again a continuously variable cone-pulley transmission of the kind that has two pairs of conical discs, i.e., a first pair mounted on a first shaft (input shaft) and a second pair mounted on a second shaft (output shaft), and an endless flexible element such as a chain or belt to transmit torque between the pulleys. The compressive gripping force of the conical discs against the chain-belt is applied through actuating members that are arranged at the disc pairs and apply a force that depends on the amount of torque to be transmitted. At least one of the disc pairs is equipped with a second actuating member, likewise a piston/cylinder unit, which is pressurized with a working fluid to a pressure level that depends on the transmission ratio that is to be set. The pressurized working fluid is directed to the ratio-controlling piston/cylinder unit by way of a transmission-ratio valve.
The transmission-ratio valve can be a pilot-controlled pressure-reducing valve whose pilot pressure is controlled by means of a proportional valve. To open up or to change the opening cross-section of a passage for a fluid stream from the transmission-ratio valve to the piston/cylinder unit, the transmission-ratio valve is controlled by the level of the pilot pressure generated by the proportional valve, so that a change in the electric current through the proportional valve leads to a change in the pilot pressure that controls the transmission-ratio valve.
Until now, the procedure for holding the transmission ratio constant in the reverse travel mode has been to use a high level of current to generate a high level of pilot pressure. This leads to a high ratio-controlling pressure in the piston/cylinder unit that controls the transmission ratio, so that the transmission is held in the slow range or underdrive with a strong actuator force. As there are unavoidable random variations in the serial production from one transmission unit to the next in regard to the amount of force that each transmission requires for being held safely in the underdrive mode, the use of a high level of current assures that any one of the different transmissions of a series can be held safely in underdrive when driving backwards.
It should be obvious that the high current level is disadvantageous to the overall fuel consumption of the vehicle and that the high level of ratio-controlling pressure or ratio-controlling force causes a high level of contact force between the conical discs and the chain-belt, which can cause increased wear on the chain-belt.
The method according to the invention therefore includes measures whereby the transmission-ratio of a cone-pulley transmission is set in such a manner that the aforementioned disadvantages are avoided.
To meet this objective, the invention provides a way of setting the transmission ratio of a continuously variable cone-pulley transmission at an essentially constant level. Under the inventive method, the transmission is first set to the targeted ratio by pressurizing at least one piston/cylinder unit, whereupon the same or another piston/cylinder unit is pressurized with a holding pressure for the purpose of maintaining the set transmission ratio. According to the inventive method, the amount of holding pressure is specific to a given transmission, i.e., the holding pressure or holding force, or a current level by which the holding pressure is generated, can be different from one unit to the next, but the amount of holding pressure is selected to be only as large as required in a given individual transmission for maintaining the set transmission ratio. The use of an unnecessarily high ratio-controlling pressure is thereby avoided.
The first step, i.e., pressurizing a piston/cylinder unit to set the transmission to the targeted ratio, can be performed with a pressure that corresponds to the holding pressure, if the holding pressure can already be used to set the transmission to the desired ratio. If a continuously variable cone-pulley transmission has more than one piston/cylinder unit for setting the transmission ratio, the ratio-setting or changing pressure can be applied to one or also to more than one of the piston/cylinder units, and the holding pressure can then be applied to another of the piston/cylinder units.
In particular, the inventive method is provided for the purpose of setting and holding the slow-speed ratio, i.e., the underdrive mode of the continuously variable cone-pulley transmission.
According to the invention, the pressure is introduced into the piston/cylinder unit by way of a transmission-ratio valve to which a controlling quantity is applied. The amount of the controlling quantity is determined such that the holding pressure is the minimum holding pressure required in a specific given transmission. The controlling quantity can be a pressure or also a current. The controlling quantity can be applied directly or indirectly to the transmission-ratio valve. With direct application, the controlling quantity is applied directly to the transmission-ratio valve. In the case of indirect application, e.g., if the transmission-ratio valve is controlled by a pilot pressure, the controlling quantity acts on the proportional valve which, in turn, generates a pilot pressure to control the transmission-ratio valve.
In an advantageous embodiment of the invention where the controlling quantity is a current, a value of the controlling current that corresponds to the minimum holding pressure is based on the difference between a controlling current that corresponds to the maximum amount of pressure and a controlling current that corresponds essentially to the zero level of the ratio-controlling pressure.
Following is a possible procedure for determining the minimum holding pressure or the corresponding amount of controlling current, which is specific to a given transmission: A test unit of the given transmission model is first set to underdrive, whereupon the engine that is coupled to the transmission is run through a sweeping variation of its rpm rate over the entire engine-rpm range while the transmission is held in a regulated underdrive condition. During the rpm sweep from idling speed to the maximum rpm rate, the controlling current will at some point reach a maximum. The maximum value of the current, as a rule, is greater than a zero-pressure value of the current. The zero-pressure value is the current value where the ratio-controlling pressure of the piston/cylinder unit is essentially zero and where the characteristic curve of the transmission-ratio valve has an inversion point. The controlling current required to hold a transmission in underdrive can be based on the difference between the maximum value and the inversion-point value of the current. To allow for the specific behavior of an individual transmission in holding the underdrive mode, the amount of controlling current required for a secure holding pressure can be determined by adding the aforementioned current difference to the inversion-point current of the given individual transmission. The controlling current determined in this manner can be significantly lower than a uniform value of the controlling current that would be large enough to cover in every case the range of random variation between all transmissions produced in a serial manufacturing process.
Among other factors, the ratio of a continuously variable cone-pulley transmission also depends on the load, i.e., the power output flowing through the transmission. It is therefore advantageous if the current value required for holding the transmission in underdrive is corrected with a load-dependent adjustment. Consequently, the current required for holding the transmission in underdrive, or also the current required for holding the transmission at the set reverse-drive ratio, by way of a pilot pressure acting on the transmission-ratio valve is no longer constant, but is composed of a load-independent portion and a load-dependent portion.
The load-dependent portion can be determined by running the transmission in a regulated underdrive mode while varying the output torque of an engine that is coupled to the transmission, whereby a load-dependent portion of the controlling current can be determined, e.g., with the engine running in drag mode (i.e., the torque-reversal mode in which the engine acts as a brake), or other values of the load-dependent portion when the torque is varied in the normal traction mode of the vehicle. The load-independent portion of the current can at first be kept at zero and changed subsequently, if the transmission is found to depart from the set underdrive condition, in which case the controlling current can, e.g., be raised by a certain amount. In reverse drive in a set underdrive ratio, it is possible to monitor the transmission either only at the beginning of the backwards motion or also repeatedly during the reverse-drive motion and to make appropriate adjustments as required in the load-independent and load-dependent portions of the current value.
In addition to the importance for the slow range, it is also essential at other transmission ratios of a continuously variable cone-pulley transmission that a set ratio is kept essentially constant. A set transmission ratio can be maintained at a constant level by setting up a counterforce acting in opposition to the ratio-changing force of a piston-cylinder unit, so that the ratio-changing force is in equilibrium with the counterforce and, therefore, no further change will take place in the transmission ratio. It is possible in this manner to establish an equilibrium between the ratio-changing force and the counterforce at any point within the entire range of ratios of the continuously variable cone-pulley transmission. Therefore, according to a further developed advantageous embodiment of the invention, the ratio of a continuously variable cone-pulley transmission is set to maintain an essentially constant value through a process in which the transmission is first set to the targeted ratio by pressurizing two piston/cylinder units and the set ratio is subsequently maintained by applying a holding pressure to at least one of the piston/cylinder units.
Under the method just described, the amount of the holding pressure is determined depending on the transmission ratio. In the slow range of the transmission, the value resulting from the ratio-dependent determination can be raised by a preset amount that is specific to the individual transmission, to hold the latter in the slow range. Thus, if the values of the holding pressure as a function of the transmission ratio are known, it is possible to establish a corresponding value for the controlling current based on the holding pressure for one piston/cylinder unit or also for both piston/cylinder units and from the characteristic curve of the transmission-ratio valve, so that the current is sufficient to securely hold the set transmission ratio, but significantly lower than a current value that would be dictated by the random variation in the serial production of transmissions with regard to the current level required for the secure holding of a set ratio.
In an advantageous embodiment of the invention, the transmission ratio is monitored, and if the transmission ratio is found to have changed, the holding pressure is raised by a preset amount. If the controlling quantity for the transmission-ratio valve is a controlling current, the latter is adjusted by a preset amount, e.g., by one percent, after a change has been detected in the transmission ratio, so that the transmission returns to the set ratio or a further change of the ratio is prevented.
The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing.